1. Field of the Invention
Apparatuses consistent with the present invention relate to a balun. More particularly, the present invention relates to a balun which reduces its size and improves its usable bandwidth.
2. Description of the Related Art
Balance to unbalance (“balun”) is a circuit which converts an unbalanced signal to a balanced signal or vice versa.
FIG. 1 is a perspective view of a conventional balun, and FIG. 2 is a cross sectional view of the conventional balun, taken along the line I-I′ of FIG. 1.
Referring to FIGS. 1 and 2, the conventional balun 90 includes a base plate 10, a ground 20, first and second output lines 30 and 40, first and second conductive portions 50 and 60, an input line 70, and a dielectric layer 80.
The ground 20 is provided under the base plate 10. The first and second output lines 30 and 40 and the input line 70 are placed on the base plate 10. The ground 20 covers all over the lower part of the base plate 10.
The first and second output lines 30 and 40 are spaced away from each other at an interval, facing each other based on the center line traversing the base plate 10. The first and second output lines 30 and 40 are patterned in a clamp shape.
At one end of the first output line 30, a first output port OP1 is provided. The first output port OP1 outputs a first output signal corresponding to an incoming signal from the input line 70. At one end of the second output line 40, a second output port OP2 is provided. The second output port OP2 outputs a second output signal corresponding to the incoming signal from the input line 70. The first and second output ports OP1 and OP2 are positioned in the proximity to each other.
The first and second conductive portions 50 and 60 electrically connect the first and second output lines 30 and 40 to the ground 20.
The first conductive portion 50 is interposed between the ground 20 and the first output line 30. The base plate 10 has a first via hole formed by removing part of it. The first conductive portion 50 is placed in the first via hole to electrically connect the ground 20 with the first output line 30, thereby electrically connecting the first output line 30 to the ground 20.
The second conductive portion 60 is interposed between the ground 20 and the second output line 40. The base plate 10 has a second via hole formed by removing part of it. The second conductive portion 60 is placed in the second via hole to electrically connect the ground 20 with the second output line 40, thereby electrically connecting the second output line 40 to the ground 20.
The input line 70 is positioned above the first and second output lines 30 and 40. The input line 70 has an input port IP at an end adjacent to the first output line 30 to receive the incoming signal from the outside.
The dielectric layer 80 is deposited on an upper surface of the base plate 10 where the first and second output lines 30 and 40 are formed. The dielectric layer 80 is interposed between the first and second output lines 30 and 40 and the input line 70.
When an unbalanced signal is input to the input port IP, the unbalanced signal is applied to the first and second output lines 30 and 40. The first and second output ports OP1 and OP2 convert the unbalanced signal to a balanced signal and output first and second output signals. Note that the first and second output lines 30 and 40 produce signals that are half of the input signal respectively.
Accordingly, the input signal is divided and output by half, and the phase difference between the first and second output signals is about 180 degrees. For so doing, the length of the input line 70 above the first output line 30 should meet about λ/4 of the input wavelength and the length of the input line 70 above the second output line 40 should meet about λ/4 of the input wavelength. Likewise, the length of the first and second output lines 30 and 40 facing the input line 70 should meet about λ/4 of the input wavelength.
To receive the unbalanced signal and output the balanced signals from the first and second output ports OP1 and OP2 of the balun 90, the input line 70 and the first and second output lines 30 and 40 are required to make their facing parts meet about λ/4.
In addition, to extend the matching frequency band, the balun 90 is required to extend the lengths of the input line 70 and the first and second output lines 30 and 40, respectively. As a result, the overall size of the balun 90 is increased.